High resolution calorimetric detection of single particles in magnetic fields up to 8 T at temperatures (0.1 K is reported. A careful study of heavily doped Si and Ge thermistors reveals an extremely large positive magnetoresistance in the variable range hopping conductivity regime. This phenomenon is used to optimize the impedance and the temperature sensitivity of a calorimetric detector. Initial experiments show an energy resolution full width at half maximum of 800 eV for 62.5 keV conversion electrons and an energy threshold of <400 eV. (C) 1996 American Institute of Physics.